Though the Franks have been planning for a move to the Annex for a while, and have been transferring belongings and supplies there in small quanitities, they have not been able to complete their preparations when the notice of Margot's deportation arrives.
The Franks and the others know that they will be in the Annex indefinitely, so they must bring us much as they can with them to last for the duration. They wear as much as they can, in anticipation that this is all the clothes they will have. The purchase of additional clothes will be possible through Miep, but requiring frequent trips to buy clothes will arouse suspicion.
Carrying luggage for clothes will also cause suspicion from the Nazis. So many people were trying to escape being deported to the concentration camps that they were constantly on guard of anyone who is carrying luggage. Being stopped would require presentation of papers, which would then identify Margot as one who is supposed to be deported. Thus the Franks are trying to avoid as much attention as possible.
they did this because since they were riding bikes they had to take as much as possible so they put on lots of clothes and rode to the secret annexe
because back then people could tell by how a person dressed what religion they wore .......plus people in certain groups or gangs dressed alike.